The concepts illustrated herein relate generally to thermal fluid ejection systems which eject precise amounts of fluid through one or more nozzles in response to a firing signal, including those used in inkjet printing mechanisms, and more particularly to a thermal monitoring system for determining whether a nozzle. is healthy.
One thermal fluid ejection system is used in inkjet printing mechanisms which have cartridges, often called xe2x80x9cpens,xe2x80x9d that shoot drops of liquid colorant, referred to generally as xe2x80x9cink,xe2x80x9d onto a page. Each pen has a fluid-ejecting printhead formed with very small, pin-hole-sized nozzles through which the ink drops are ejected. To print an image, the printhead is propelled back and forth across the page, shooting drops of ink in a desired pattern as it moves. Two earlier thermal ink ejection mechanisms are shown in U.S. Pat. Nos. 5,278,584 and 4,683,481, both assigned to the present assignee, Hewlett-Packard Company. In a thermal system, a barrier layer containing ink channels and vaporization or firing chambers is located between a nozzle orifice plate and a substrate layer. This substrate layer typically contains linear arrays of heater elements, such as resistors, which are energized to heat ink within the vaporization chambers. Upon heating, an ink droplet is ejected from a nozzle associated with the energized resistor. By selectively energizing the resistors as the printhead moves across the page, the ink is expelled in a pattern on the print media to form a desired image (e.g., picture, chart or text).
Non-functioning nozzles in the context of an inkjet printer may contribute to print quality defects when trying to print a desired image on a sheet of media, such as paper, and when dispensing other fluids, non-functioning nozzles result in an inadequate amount or inaccurate placement of the fluid on the receiving surface. There are a variety of possible causes for non-functioning nozzles, including: (1) internal jetting head contamination; (2) vapor bubbles within the jetting head; (3) crusting of the fluid over the nozzles; (4) external jetting head contamination; and (5) resistors which fail to fire. Other causes for non-functioning nozzles may exist, depending upon the particular implementation. Various schemes have been proposed to replace non-functioning nozzles with functioning nozzles in multipass fluid ejection routines or print modes, for instance, by using backup nozzles to help restore some of the fluid placement quality lost by the bad nozzles. These various fluid ejection routines or print mode schemes rely on the ability to reliably detect and determine when a nozzle is not functioning.
Unfortunately in the inkjet printing context, the combination of small nozzles and quick drying ink leaves the printheads susceptible to clogging, not only from dried ink and minute dust particles or paper fibers, but also from the solids within the new inks themselves. Partially or completely blocked nozzles can lead to either missing or misdirected drops on the print media, either of which degrades the print quality. Nozzle xe2x80x9cspittingxe2x80x9d routines eject ink to push dried ink clogs into a waste receptacle, referred to as a xe2x80x9cspittoonxe2x80x9d in the art. Besides merely forcing clogs out of the nozzles, spitting also heats the ink near the nozzles, which decreases the ink viscosity and assists in dissolving ink clogs.
Air bubbles lodged within the printhead may also prevent a nozzle from firing. These air bubbles may be pulled by a vacuum force from the printhead in a priming routine, such as that taught in U.S. Pat. Nos. 5,592,201 and 5,714,991, both assigned to the present assignee, the Hewlett-Packard Company. In devices which are not equipped with a priming system, the air bubbles may be pushed out of the printheads by applying a positive force to the ink reservoir supplying the printhead. For instance, an inkjet pen body may serve as an ink containment reservoir that protects the ink from evaporation and holds the ink so it does not leak or drool from the nozzles. Ink leakage is prevented using a force known as xe2x80x9cbackpressure,xe2x80x9d which is provided by the ink containment system. Desired backpressure levels may be obtained using various types of pen body designs, such as resilient bladder designs, spring-bag designs, and foam-based designs. By applying a force to the ink contained in these reservoirs, the ink itself may be used to push the air bubbles out of the nozzles.
In operating a precision fluid ejection system, such as an inkjet printing mechanism, it would be helpful to provide feedback to a print controller, such as a printer driver residing in an on-board microprocessor and/or in the host computer, as to whether or not the printhead nozzles are firing as instructed. This information would be useful to determine whether a nozzle had become clogged and required purging or spitting to clear the blockage. This information would streamline the spitting process and conserve ink because only the clogged nozzle(s) would be spit to clear the blockage. Moreover, if damaged nozzles or heating elements could be detected, then other nozzles may be substituted in the firing scheme to compensate for the damaged nozzles.
A variety of different schemes have been used to detect a failed nozzle. For example, a failed firing resistor may be detected by a special circuit in the printer that looks at the resistance of the drive circuit, and if the resistance indicates an open circuit then clearly the resistor will not fire because it cannot receive a firing pulse. Various sensors have been used in the past to detect whether a droplet has been ejected from a nozzle. For example, in one method a photo-diode and a light emitting diode (LED) pair are used to detect the shadow of a droplet passing between the photo-diode and the LED. One optical system measured the change in drop volume for a given firing temperature by firing smaller and smaller droplets until the drops could no longer be seen by an optical detector. Unfortunately, the target drop volume has decreased in newer inkjet cartridges, with some droplets now being on the order of 5 picoliters. These small droplets require either multiple firings to increase the signal or precise positioning of such an optical drop detector, which is difficult to implement consistently and reliably in production printing mechanisms.
In another system, a piezo electric film is used as a droplet target to detect whether or not a droplet impacts the target. In an electrostatic detection method, the positive or negative charge from an ejected droplet is detected. In yet another method, piezo-electric crystals are used to detect the acoustic signature generated as a droplet is ejected from the printhead. All of these methods have been built and tested, at least in a prototype environment, and have been found to effective at detecting nozzle outages, and in some cases, even weak or misdirected droplets.
Unfortunately, all of these earlier detection methods suffer two severe shortcomings. First, these earlier methods are unable to detect nozzles outages xe2x80x9con-the-flyxe2x80x9d during normal fluid ejection activities, such as during printing. Second, these earlier methods are unable to detect nozzle outages at the full firing frequency of the jetting head. This inability to detect non-functioning nozzles on-the-fly during a print job or other fluid ejection activity may lead to serious problems, because nozzle health may change during any fluid ejection routine or print job. Since nozzles may fail on-the-fly, it would be desirable to have a nozzle replacement system which detects non-functioning nozzles on-the fly, and applies a correction system to utilize replacement nozzles on-the-fly so the resulting fluid ejection or print job occurs as originally intended with high quality.
According to one aspect of the present invention, a method is provided for monitoring the health of a fluid ejection nozzle which normally ejects a fluid in response to a firing signal. In this method several things occur, including: applying a firing signal to said nozzle; then thereafter, monitoring the temperature change of the nozzle; and finally determining from the monitored temperature change whether the nozzle ejected said fluid in response to the application of the firing signal.
According to another aspect of the invention, a fluid ejection mechanism is provided as including a fluid reservoir containing a fluid, and a fluid jetting head having a nozzle in fluid communication with the reservoir to receive the fluid and normally, in response to a firing signal, eject said fluid through this nozzle. Unfortunately, sometimes the nozzle is in xe2x80x9cpoor healthxe2x80x9d being clogged or blocked and unable to eject the fluid when asked. To address this issue, the fluid ejection mechanism also has a temperature sensor which monitors temperature change of said nozzle and generates a temperature signal in response to this change. The fluid ejection mechanism also has a controller which generates the firing signal. The controller also determines from the temperature signal whether the nozzle ejected the fluid in response to the application of the firing signal.
According to another aspect of the invention, a fluid ejection mechanism is provided with a fluid reservoir containing a fluid, and a fluid jetting head. The head has a nozzle which is in fluid communication with said reservoir to receive the fluid and normally, in response to a firing signal, eject the fluid through the nozzle. The fluid ejection mechanism also has means for applying the firing signal to said nozzle, and means for monitoring the temperature change of the nozzle. The fluid ejection mechanism also has a means for determining from the monitored temperature change whether the nozzle ejected the fluid in response to the application of the firing signal.
An overall goal herein is to provide a monitoring system for determining on-the-fly whether a thermal, fluid-ejecting nozzle is healthy during a firing routine without unnecessary interruption, and for employing nozzle recovery or replacement routines when unhealthy nozzles are found.
Another goal herein is to provide a thermal monitoring system for monitoring printhead nozzle health when installed in an inkjet printing mechanism.